The present invention relates to a constant-velocity joint through which a driving shaft is connected to a driven shaft to transmit torque from the driving shaft to the driven shaft, and an image forming device such as a copier or a laser beam printer (LBP) in which a constant-velocity joint is used.
A known image forming device includes a photoconductor drum which is rotated in one direction. Around the photoconductor drum, an electrifier, an exposure unit, a developing unit and a transfer/electrifier are provided. The electrifier uniformly charges the entire outer periphery of the photoconductor drum. Image light based on image information is thrown onto the thus charged outer periphery of the photoconductor drum by the exposure unit to form an electrostatic latent image thereon. Toner is supplied onto the latent image from the developing unit to form a toner image. The toner image is transferred onto a transfer material being fed at the same speed as the peripheral speed of the photoconductor drum by the transfer/electrifier.
In the case of a monochromatic image forming device, images are formed by a single image forming unit comprising a photoconductor drum, electrifier, exposure unit, developing unit and transfer/electrifier. A full-color image forming device includes four such image forming units that are arranged in tandem in the direction in which transfer materials are fed. The four image forming units form yellow, magenta, cyan blue and black toner images, respectively. By transferring these toner images onto a transfer material, a color image is formed on the transfer material.
In either a monochromatic or a full-color image forming unit, because images are formed by rotating the photoconductor drum or drums, if the photoconductor drum is not rotated at a constant speed, electrostatic images formed on the photoconductor drum by exposure with the exposure unit expand or shrink. Images also expand or shrink when transferring toner images onto a transfer material. Such expansion and shrinkage of images deteriorates the quality of images. It is therefore important to rotate the photoconductor drum at a constant speed.
In a known image forming device, the rotary shaft of a motor is directly connected to the shaft of the photoconductor drum (hereinafter referred to as “drum shaft) to rotate the photoconductor drum. In this arrangement, due to mounting errors when supporting the photoconductor drum and mounting the motor, it is extremely difficult to precisely align the drum shaft and the rotary shaft of the motor. If the drum shaft and the rotary shaft are not aligned with or inclined relative to each other, it is impossible to rotate the photoconductor drum at a constant speed and thus to obtain high-quality images.
In order to solve this problem, JP patent publication 5-341589A proposes to couple the drum shaft to the rotary shaft of the motor through a joint, and to arrange the exposure position and the image transfer position on the photoconductor drum so as to be diametrically opposite to each other. With this arrangement, it is allegedly possible to prevent expansion or shrinkage of images on a transfer material by canceling any expansion or shrinkage of images at the exposure position due to nonuniform rotational speed of the photoconductor drum with the shrinkage or expansion of the images at the transfer position.
But because the joint used in the image forming device disclosed in JP patent publication 5-341589A is a nonconstant-velocity joint, it is impossible to rotate the photoconductor drum at a uniform speed. Thus, even if the exposure position and the transfer position of the photoconductor drum are arranged so as to be diametrically opposite to each other, it is impossible to completely cancel out expansion and shrinkage of images because the diametrically opposed positions tend to shift due e.g. to assembling errors or inclination or deflection of the photoconductor drum, which in turn makes it impossible to form high-quality images.
Another problem is that the input and output members of the non-constant-velocity joint are not easily disassemblable, so that it is troublesome to replace parts of the image forming device such as the photoconductor drum. Maintenance is thus difficult.
Sliding constant-velocity joints are known, which permit angular and axial displacements of the input and output shafts.
For example, JP patent publication 3058213 discloses a constant-velocity joint comprising an outer ring having a central axis and formed with a bore defined by six flat surfaces that are parallel to the central axis and having a hexagonal cross-section, and a trunnion member received in the bore of the outer ring, the trunnion member having an outer periphery formed with three first spherical surfaces circumferentially spaced apart from each other and three second spherical surfaces circumferentially spaced apart from each other and each disposed between adjacent ones of the first spherical surfaces, each of the first spherical surfaces being in contact with one of the flat surfaces at a point offset from a circumferential center of the flat surface in one circumferential direction of the outer ring, each of the second spherical surfaces being in contact with another of the flat surfaces at a point offset from a circumferential center of the flat surface in the other circumferential direction. Torque is thus transmitted between the outer ring and the trunnion member.
This constant-velocity joint comprises a small number of parts, and thus can be made small and lightweight. Because the spherical surfaces of the trunnion member are in sliding contact with and guided by the flat surfaces of the outer ring, angular and/or axial displacement of the outer ring relative to the trunnion member is made smoothly.
The constant-velocity joint disclosed in JP patent publication 3058213 is mainly used for motor vehicles and thus has to have sufficient strength. Its outer ring and trunnion member are therefore both made of a metal such as steel.
Although constant-velocity joints entirely made of a metal have sufficient strength, they are heavy, need lubrication with grease, and produce much noise during operation. Thus, applications of such joints are limited. For example, use of such joints in office machines, audio devices, medical devices and instruments and household appliances will be problematic.
Use of such joints in food processing devices machines is also problematic because in order to prevent foodstuffs from being contaminated with leaked lubricating grease, it is necessary to provide the joints with means for preventing leakage of grease such as a boot. Such means naturally increases the number of parts and the size of the joints and pushes up their cost.
If such a constant-velocity joint is used in an image forming device, the boot has to be mounted and dismounted when assembling and disassembling the joint. This makes maintenance of the image forming device difficult.
An object of the present invention is to provide a constant-velocity joint which needs no grease lubrication, which is lightweight and small in size, which produces less noise during operation, and which can be used for a large variety of machines and devices, and an image-forming device which includes means for rotating the photoconductor drum always at a uniform speed so that high-quality images can be formed, and which is easy in maintenance.